1. Field of the Invention
The present invention relates to sound damping devices that dampen noise in running vehicles, and in particular to standing wave attenuation devices that attenuate standing waves in cabins or rooms of vehicles.
The present application claims priority on Japanese Patent Application Nos. 2010-235833 and 2011-196777, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
In general, vehicles suffer from vibrations of the wheels while running, which are transmitted into cabins or rooms of vehicles, thus causing noise with a broad range of frequency components. This noise is called road noise, which is transmitted into cabins or rooms of vehicles to cause standing waves offensive to human cars. Patent Document 1 discloses a technology for attenuating standing waves in cabins or rooms of vehicles. Patent Document 1 discloses in conjunction with its FIGS. 15 to 18 that a plurality of pipes, each having a quarter length of each standing wave, is fixed to the interior surface of a roof inside a cabin of a vehicle. When standing waves whose frequencies match the resonance frequencies of pipes occur in a cabin of a vehicle, a pipe resonating phenomenon occurs in pipes so as to cancel out energy of standing waves. Thus, this technology is able to attenuate standing waves in a cabin of a vehicle.
The technology of Patent Document 1 needs to determine lengths of pipes, which are sufficient to attenuate standing waves in cabins of vehicles, based on dimensions of cabins in advance, whereby these pipes are fixed under roofs of vehicles. Vehicles such as four-door sedans, for example, provide cabins whose shapes may easily cause standing waves with frequencies around 160 Hz. Long pipes whose lengths are 50 cm or more should be prepared to attenuate standing waves at 160 Hz by way of the pipe resonating phenomenon. However, it is difficult to install long pipes inside cabins of vehicles. Even if long pipes are successfully installed in cabins, they may give a sense of oppression to drivers or passengers in vehicles. When vehicles undergo fluctuations in vibration directions and frequencies due to age degradation in excitation conditions such as air pressures applied to tires of wheels, it is difficult to adapt to fluctuating vibration conditions by way of resonance frequencies of pipes; hence, it becomes difficult to attenuate standing waves in cabins over a lapse of time.